1. Field of the Invention
The present invention relates to a method of manufacturing a rotation detector having a biasing permanent magnet and a magnetic sensor chip that are accommodated in a protecting cap.
2. Description of the Related Art
JP-A-2007-101230 discloses a conventional rotation detector 90 having a biasing permanent magnet and a magnetic sensor chip that are accommodated in a protecting cap.
Such a rotation detector is mounted in an internal combustion engine to detect rotation of an engine crankshaft.
The rotation detector includes a semiconductor chip (bare chip) 10, a case body 20, a biasing permanent magnet 30 and a cap 40. The rotation detector, as a whole is almost the same as a rotation detector 90 according to a preferred embodiment of the invention shown in FIG. 2A except for a portion encircled by a broken line and indicated by reference numeral 1C, which will be described later. The magnetic sensor chip 10 and the biasing permanent magnet 30 are accommodated in the cap 40 to be protected from ambient atmosphere.
The semiconductor chip 10 includes a sensing chip 11 on which a pair of magnetoresistance elements (not shown) is formed and a processor chip 12 that processes the output signals of the sensor chip. The case body 20 is made of resinous material and has a disk-shaped base portion 25 and a tongue portion 21 that projects from the base portion. The sensing chip 11, the processor chip 12 and a lead frame are interconnected and embedded in the tongue portion 21.
The biasing permanent magnet 30 is a cylindrical member having a longitudinal rectangular cavity 31, in which the semiconductor chip 10 and the tongue portion 21 are disposed. The biasing permanent magnet 30 gives the magnetoresistance elements a biasing magnetic field.
The cap 40 is a non-magnetic cylindrical member that has a generally conical brim 41 at an open end thereof and a bottom at the other end. The brim 41 has a conical inside surface 41t that expands wide toward the joint surface 25t from the open end 41, and the base portion 25 is formed to have a generally tapering joint surface 25t. The brim 41 is fixed to the base portion 25. Thus, the sensing chip 11, the tongue portion 21 and the biasing permanent magnet 30 are protected from ambient atmosphere.
This rotation detector is disposed near a portion of an engine crankshaft so that the right end thereof confronting the portion of the engine crankshaft. When the crankshaft rotates, the biasing magnetic field that is given by the biasing magnet changes, so that the sensing chip provides signals, which are processed by the processor chip in a prescribed manner to detect a rotation angle, a rotation speed, etc.
In manufacturing, the case body 20, the biasing permanent magnet 30 and the cap 40 are assembled in a prescribed manner, and the brim 41 of the cap 40 is pressed against the base portion 25 so that the conical inside surface 41t can be brought into close contact with the tapering joint surface 25t. Thus, the cap 40 is positioned coaxial with the case body 20, as shown in FIG. 3A. In the meantime, the biasing permanent magnet 30 is urged by the bottom of the cap 40 so that the inside surface thereof can be fitted to the base portion 25.
Thereafter, the brim 41 of the cap 40 and the base portion 25 of the case body 20 is welded by a laser welder. In more detail, a laser beam is irradiated at the outside surface of the brim 41 (behind the inside surface 41t) to melt the tapering joint surface 25t of the base portion 25 and the inside surface 41t of the brim 41. Accordingly, a mixture of melted materials of the base portion 25 and the brim 41 is produced in the boundary area between the joint surface 25t and the inside surface 41t and also at the circumference of the brim 40, as shown in FIG. 3B. When the laser welding is stopped, the melted materials cools down and starts to re-crystallize, so that the cap 40 and the case body 20 are firmly fixed.
However, it has been sometimes found that an uneven joint area 50 and burrs or fins 60 shown in FIG. 3C.
As a result, the uneven joint area 50 distorts the tapering surface 25t of the base portion 25, so that the axis of the cap 40 may diverge from the axis of the case body 20, and the burrs and fins 60 may ruin the coaxial relation of cap 40 with the case body 20. This may cause an inaccurate detection of rotation.